New research on Auckland’s
volcanic field has uncovered a volcano which had been all
but forgotten, and this work will better define what is
most likely to happen when the next volcano forms in
Auckland. The research is part of the Devora project, a
seven-year study to better understand the volcanic history
of Auckland and help prepare the city for a future
eruption.

By analysing records from boreholes drilled for
foundations of buildings, roads and or for water supply,
scientists have been able to identify a previously
little-known volcano now hidden beneath the suburb of
Grafton, close to the Auckland University Medical School.

Geologist Bruce Hayward, of Geomarine Research, put
together the volcanic puzzle by linking lava flows between
boreholes and measuring changes in the thickness of the
lava flows and volcanic ash it is possible to identify a
buried volcanic crater.

“The crater is about 1km across
and filled with solidified lava flows,” Dr Hayward said.
Auckland is built on the Auckland Volcanic Field, a group
of about 50 volcanoes that have erupted over the last
250,000 years. Scientists believe that most of the
volcanoes erupted only for a few months or years and then
became inactive. However, our knowledge of exactly when
each volcano erupted, and how future eruptions might
progress is incomplete.

Close inspection of the second
oldest geological map of the Auckland Volcanic Field,
published by early explorer and geologist Ferdinand von
Hochstetter in 1864, shows four volcanic vents in the
vicinity of the Domain.

“So although this find is
exciting, it is clear that Hochstetter recognised the
presence of a volcano in this locality before it was
covered in houses,” Dr Hayward said.

“It would appear
that this Grafton volcano erupted just before the
neighbouring Domain Volcano, more than 50,000 years ago.”
A thick layer of volcanic ash from the Domain eruptions
buried and hid the Grafton Volcano until recent boreholes
have shown its full extent and nature.

Over the last few
years, geophysicists at The University of Auckland have
also been studying the rocks under Auckland using gravity
and magnetic measurements. Because lava is very magnetic,
airborne surveys of the city have revealed where lava is
present underground, even where it is not visible at the
surface.

One area of high gravity and magnetism is in the
same area that Dr Hayward has identified the new Grafton
volcano. Bringing together these different methods has
helped to confirm the findings.

Joint Project Leader, Dr
Jan Lindsay from The University of Auckland, said the new
information shows that we still have a lot to learn about
the past volcanic activity in Auckland.

Scientists hope to
bring the borehole data from across the city into a central
database where it can be used to model where past eruptions
have occurred.

The Devora project is led jointly by GNS
Science and The University of Auckland in collaboration
with Massey University and brings together data and
researchers from many different areas of study. The
project is funded jointly by the Earthquake Commission, the
Auckland Council, the Ministry of Science and Innovation and
The University of Auckland. Project Devora, which stands
for Determining Volcanic Risk in Auckland, started in late
2008.

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